1. Field of the Invention
The present invention relates to a television tuner capable of receiving a UHF-band television signal and a VHF-band television signal.
2. Description of the Related Art
The structure of a known television tuner is illustrated in FIG. 2. A VHF- or UHF-band television signal is input to an input terminal 1. A VHF input tuning circuit 3 capable of switching bands is connected to the input terminal 1 via a filter 2. A UHF input tuning circuit 5 is connected to the filter 2 via a switching diode 4. A voltage from the VHF input tuning circuit 3 is applied to the anode of the switching diode 4. The output terminal of the VHF input tuning circuit 3 and the output terminal of the UHF input tuning circuit 5 are connected to an integrated circuit 20. First and second dual-gate field-effect transistors (FETs) 21 and 22, a third single gate FET 23, and a plurality of resistors are disposed inside the integrated circuit 20, wherein the resistors mutually connect the first, second, and third FETs 21, 22, and 23.
Inside the integrated circuit 20, a first gate G1 of the first FET 21 and a drain D of the third FET 23 are connected by a first resistor 24, a second gate G2 of the first FET 21 and a second gate G2 of the second FET 22 are connected by a second resistor 25, a source S of the first FET 21 and a source S of the second FET 22 are connected by third resistor 26, and a gate G of the third FET 23 and the first gate G1 of the second FET 22 are connected by a fourth resistor 27.
The integrated circuit 20 includes a first external terminal 20a connected to the first gate G1 of the first FET 21, a second external terminal 20b connected to the second gate G2 of the first FET 21, a third external terminal 20c connected to the drain D of the third FET 23, a fourth external terminal 20d connected to the first gate G1 of the second FET 22, a fifth external terminal 20e connected to the source S of the second FET 22, a sixth external terminal 20f connected to a drain D of the second FET 22, a seventh external terminal 20g connected to the source S of the first FET 21, and an eighth external terminal 20h connected to a drain D of the first FET 21.
The VHF input tuning circuit 3 is connected to the first external terminal 20a, and the UHF input tuning circuit 5 is connected to the fourth external terminal 20d. The second external terminal 20b is connected to an automatic gain control (AGC) terminal 8. An AGC voltage is applied to the AGC terminal 8.
One end of a peaking coil 9 is connected to the fourth external terminal 20d. The other end is connected to a direct current (DC) cutting capacitor 10 for high-frequency grounding and is also connected to a switching terminal 11. The level of the switching voltage applied to the switching terminal 11 increases when the switching terminal 11 receives a UHF-band television signal and decreases when the switching terminal 11 receives a VHF-band television signal.
The third external terminal 20c is connected to the cathode of the switching diode 4 via a resistor 12 and is connected to a power terminal 14 via a pull-up resistor 13.
The eighth external terminal 20h is connected to a VHF inter-stage tuning circuit (not shown in the drawing). A power-supply voltage is applied to the VHF inter-stage tuning circuit and is further applied to the first FET 21 via the eighth external terminal 20h. The seventh external terminal 20g is directly grounded.
The sixth external terminal 20f is connected to a UHF inter-stage tuning circuit (not shown in the drawing). A power-supply voltage is applied to the UHF inter-stage tuning circuit and is further applied to the drain D of the second FET 22 via the sixth external terminal 20f. The fifth external terminal 20e is grounded by a capacitor 15.
According to the above-described structure, when the television tuner receives a UHF-band television signal, a predetermined bias voltage required for operation is applied to the first gate G1 of the second FET 22 by applying a high-level switching voltage to the switching terminal 11. In this way, the television tuner enters an operational state. Simultaneously, the third FET 23 turns on, causing the switching diode 4 to turn on. Consequently, the television signal can be input to the UHF input tuning circuit 5. The television signal selected by the UHF input tuning circuit 5 is input to the first gate G1 of the second FET 22.
When the third FET 23 turns on, the voltage at the first gate G1 of the first FET 21 becomes a bias voltage, and, thus, the television tuner enters a not-ready state.
When the television tuner receives a VHF-band television signal, a low level switching voltage is applied to the switching terminal 11. Accordingly, the bias voltage at the first gate G1 of the second FET 22 becomes zero, and the second FET 22 enters a not-ready state. Moreover, since the third FET 23 turns off, the switching diode 4 also turns off. Thus, a television signal is not input to the UHF input tuning circuit 5. Since the third FET 23 is turned off, a predetermined bias voltage required for operation is applied to the first gate G1 of the first FET 21, enabling the operation of the first FET 21. Although not shown in the drawing, a resistor for adjusting the bias voltage to a correct value is connected between the first gate G1 of the first FET 21 and the source S of the first FET 21. A VHF-band television signal selected by the VHF input tuning circuit 3 is input to the first gate G1 of the first FET 21 and is amplified (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-368639).
According to the above-described known structure of a television tuner, the bias voltages applied to the first gate G1 of the first FET 21 and the first gate G1 of the second FET 22 are determined by the resistors disposed inside the integrated circuit 20. Therefore, the values of the voltages are predetermined and the gain reduction versus AGC voltage characteristics (i.e., AGC curve) cannot be changed.